1. Field of the Invention
The present invention relates to a SOG (Spin-On-Glass, hereinafter referred as SOG) dispensing system and its controlling sequences and, more particularly, to a SOG dispensing system allowing for continuous production and raising its operating efficient.
2. Description of the Prior Art
Before we start to introduce a SOG dispensing system of prior art, it is necessary to explain importance of SOG for a semiconductor device and its fabrication processes. It is well known that uneven (non-planar) surface topology of a wafer can cause undesirable effects and difficulties in the application of subsequent layers and fabrication processes such as pattern distortion after developing due to a focusing problem in a photolithography process, it becoming more serious at a recessed area because of insufficient exposure, Especially at a multilevel metallization structure, an interlevel dielectric (hereinafter referred as ILD ) is not only used as a dielectric layer but a planarizing layer to eliminate voids formed at gaps between metal conduction lines. During a photolithography process, if there are voids formed, photoresist would flow into these voids and thus the thickness of photoresist isn't thick enough to protect the ILD layer during etching process and causes a quality-controlled issue of ILD. Besides, it is hard to clear photoresist inside the voids and thus causes a wafer-contaminated problem so as to reduce production yield. In order to overcome said problem, SOG is dispensed to the wafer for planarizing its surface topology because SOG has a flowing characteristic like a photoresist to fully filling up gaps between metal conduction lines. The wafer is then baked so that organic solvent inside the SOG can be evaporated to make it transform from a fluidity to a solidity.
A SOG dispensing system of prior art, shown in FIG. 1, comprises a source tank 10 in which contains a SOG bottle 11 having a volume 500 cc, and a coating unit 20. The reason why we use the SOG bottle is that life time of SOG chemical is maintained at a temperature between -1.degree. C. and 4.degree. C. and reduced to about 10 days at room temperature so that a SOG bottle is easy to be replaced when used up. Otherwise, a cooling unit is needed to provide low temperature and this is not ecnomical. The coating unit 20 comprises a valve V1 and a valve V2 conncected to the valve V1, wherein the valve V1 is controlled by a timing circuit (not shown in FIG. 1) to dispense a certain amount of SOG to a wafer and the valve V2 is controlled by a detector (not shown in FIG. 1) to suck back residual SOG at nozzle of its output pipe line. SOG is pushed by a high pressure of N.sub.2 and transmitted from the SOC bottle 11 through the valve V1, the valve V2 to nozzle of its output pipe line, finally dropping to the wafer W,as shown in FIG. 1. According to practical situation of production line, about 100 cc-200 cc of SOC is left after finishing SOG coating of three lots of wafers (i.e. 150 wafers) and in order to avoid process discontinuity for a next lot of wafers due to insufficient SOC, the residue can't be used and the SOG bottle should be replaced. Thus, the SOG dispensing system must be idle when changing the SOG bottle 11, thereby prolonging fabrication cycle time, as well as raising fabrication cost because of unused residue of the SOG bottle. Since in order to issure that there are no bubbles or particles in these dispensing lines, a dummy dispensing of a test running for particles and thickness confirmation is necessary after changing the SOG bottle, idle time of the SOG dispensing system is further prolonged and thus considerably lowers its operating efficient. Therefore, it is needed to propose a new SOG dispensing system allowing for continuous production, thereby raising its operating efficient and reducing fabrication cost by saving SOG waste .